Current available sealing methodologies for the mechanical stirring of vessels under high pressure is cumbersome, unreliable and does not have long life. This has always been a problem in the pharmaceutical and chemical industry and a number of users have used magnetic stirrer systems to eliminate this problem. However, magnetic stirrer systems are not always convenient and cannot be used in many instances either due to incompatibility to vessel material or due to inaccessibility.
Traditional design that does not use magnetic stirrers uses o-rings and packings as seal mechanisms for the stirrers. These types sealing mechanisms require additional forces to be exerted on them to provide the seal. This force increases with increasing operating pressure. The high force on a rotating shaft leads to a tenuous seal life. To try alleviate this problem, designers have developed multiple seal designs. However, this benefits comes with a major disadvantage: seal replacement has become a very tedious job. This problem is obvious from the lack of any competitively priced mechanical stirrers for pressures greater than 1500 psi and especially for pressures greater than 5 or 10,000 psi.
Replacing the o-rings or packings with a special energized seals such as the spring loaded seals that are similar to the ones used in HPLC pumps, provide an effective sealing method requiring very minimal force. It is practically "self-sealing". The seal consists of a polymeric body with a metal spring or a o-ring in the middle. The spring pushes the two lips of the seal outside. One side of the seal touches the body of the vessel and the other side of the seal touches the cap. When the pressure inside the vessel increases, it acts against the seal and pushes the lips even further towards the wall. This type of a seal requires very little force to maintain a seal.
The spring loaded seal is more forgiving on the stirrer shaft than the other seals. This allows the design to incorporate a little misalignment on the stirrer shaft and thus a more robust sealing mechanism. A smoother finish on the stirrer shaft (especially where the spring loaded seal is) also increases the life of the seal. This can be accomplished in many ways: a polished shaft, a polished ceramic shaft, or a ceramic coated shaft.
The stirrer shaft can be rotated through a number of ways. Connecting to an electric motor directly would be the simplest. By controlling either the voltage or the current or both, the stirrer speed can be controlled. The electric motor can be replaced with an air motor. This would be especially important in explosion proof environments.
A high pressure stirring system can increase the rate of extraction and rate of reactions under various conditions. For instance, in supercritical or subcritical fluid extraction, stirring has shown to have improved reaction rates by numerous personnel in the area of natural product extraction, precision cleaning using supercritical carbon dioxide and reactions under supercritical conditions.